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Endogenous and exogenous cannabinoids signal through the cannabinoid 1 receptor (CB1R) to modulate various aspects of social behavior, including aggression and anxiety. In rodents and primates, CB1R expression in the basolateral amygdala is dense and cannabinoid signaling in this region has been reported to influence social behavior. Little is known about how endocannabinoid signaling in the amygdala contributes to primate social diversity. The behaviorally diverse and species-rich cercopithecoid monkey genus Macaca is an ideal model for addressing this topic. Japanese (Macaca fuscata) and rhesus macaques (M. mulatta) display similar social styles in some respects; however, there is evidence to suggest they differ in their stress response, amygdala structure, and monoaminergic signaling. To further assess the molecular basis of social style in Japanese and rhesus macaques, we used immunohistochemistry and stereological methods to compare CB1R-immunoreactive (CB1R-ir) axon density in the basolateral amygdala, which is comprised of the lateral, basal, and accessory basal nuclei. Our study sample included 6 Japanese and 5 rhesus macaques. Repeated-measures ANOVAs were used to evaluate species differences, with amygdala region as the within-subjects measure and species as the between-subjects factor. This revealed significant main effects for species and area (p values < 0.05) with no interaction. Post hoc tests revealed higher CB1R-ir axon density in the basal and accessory basal nuclei of rhesus macaques compared to Japanese macaques. Our results suggest that CB1R-mediated signaling in the lateral nucleus of the amygdala is comparable between the two species, while the differences we observed in the basal and accessory basal nuclei may contribute to the nuanced behavioral differences observed between them.
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A comparison of cannabinoid receptor 1-immunoreactive axon density in the amygdala among nine primate species
The amygdala is a sensory integration center that plays an important role in emotional learning, behavior, and motivation. Cannabinoid signaling in the amygdala modulates aspects of anxiety, aggression, and fear in rodents via cannabinoid receptor 1, however little is known about cannabinoid signaling in the amygdala of humans and nonhuman primates. Primates are behaviorally diverse, with closely related species often displaying distinct social styles characterized by varying degrees of social tolerance and agonistic tendencies. Such behavioral differences are thought to be associated with neurochemical differences among species. Given what is known about the functional role of cannabinoid signaling in the amygdala, we tested whether relatively tolerant species, such as humans, bonobos, and marmosets, possess relatively higher cannabinoid receptor 1-immunoreactive (CB1R-ir) axon density in the basolateral amygdala. We used immunohistochemistry and stereological methods to compare CB1R-ir axon density among 47 primates representing nine species: humans (n=5), chimpanzees (n=6), bonobos (n=2), baboons (n=6), rhesus macaques (n=5), Japanese macaques (n=6), pigtail macaques (n=6), marmosets (n=5), and capuchins (n=6). The basolateral amygdala is comprised of the lateral, basal, and accessory basal nuclei. Stereological data for each nucleus was collected separately. After ruling out sex differences within each species, we used repeated measures ANOVA to evaluate species differences. The interaction (F16,76 = 5.061, p<.001) and main effects of species (F8,38 = 8.007, p<.001) and area (F2,76 = 59.616, p<.001) were all significant. However, the observedspecies differences did not support our hypothesis related to social tolerance nor did the data conform to a phylogenetic pattern. Instead, we found that while some closely related species differed from each other in a nucleus-dependent manner, some distantly related species shared unexpected similarities. Our results highlight the need for additional comparative work on the cannabinoid system from a molecular and genetic perspective. We discuss the implications of our observations with special focus on primate brain evolution and its connection to primate social style.
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- Award ID(s):
- 1846201
- PAR ID:
- 10397436
- Date Published:
- Journal Name:
- Abstracts Society for Neuroscience
- ISSN:
- 0190-5295
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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